Embodiments of the present invention generally relate to controlling the indoor airflow for heat pump systems. More particularly, embodiments of the present invention relate to controlling a leaving air temperature when the heat pump is operating in a heating mode.
When heating systems operate in dwellings, the temperature of the air entering the occupant zone has a significant impact on occupant comfort. If the air is not of the proper temperature, many occupants will experience some level of discomfort. Certain conventional heat pump systems have constant airflow rates based on the nominal heating capacity delivered. Nominal heating capacity is generally gauged at an outdoor temperature of 47° Fahrenheit and with an ice-free outdoor heat exchange coil.
As the outdoor temperature drops and/or frost develops on the outdoor heat exchange coil, the actual heating capacity of the system typically drops. Further, the temperature of the air leaving the heat pump condenser may depend upon the heat pump heating capacity, the temperature of the air entering the condenser, and the airflow across the condenser coil. This relationship can be expressed as:
      T    L    =            T      E        +          Q                        V          .                *        c            wherein TL is the temperature of the air leaving the condenser (for example, to be discharged to an occupied space), TE is the temperature of the air entering the condenser (for example, recirculated from the occupied space), Q is the condenser heating capacity of the system, {dot over (V)} is the volumetric airflow rate, and c is a constant combining unit conversions and specific heat of dry air. As is evident from the above equation, when the heating capacity (Q) of the system drops, so does the leaving air temperature (TL), assuming a fixed airflow rate. Should the heating capacity (Q) drop too low, the leaving air temperature (TL) may fall to a temperature that many occupants may consider uncomfortable. This problem can be compounded as ice forms on the outdoor heat exchange coil, further decreasing the heating capacity (Q) of the system.
Some conventional systems provide a variable airflow based on outdoor ambient temperature. For example, a traditional system may monitor the ambient outdoor temperature, and when the ambient outdoor temperature changes, the speed of the indoor fan may be adjusted so as to maintain the supply air temperature at a constant level over the operating range of the unit. Yet, a shortcoming of such an approach is that as ice forms on the outdoor heat exchange coil, the heating capacity of the system diminishes. Yet, while the heating capacity of such systems is decreased due to the formation of ice on the coil, because the monitored outdoor ambient temperature generally remains the same, the airflow can remain unchanged. As a consequence, the leaving air temperature (TL) may drop, leading to reduced occupant comfort.
As is evident from the foregoing, present heat pump systems suffer from a variety of disadvantages and problems including, among others, those respecting control of the leaving air temperature. There is a need for the unique heat pump control apparatuses, systems and methods disclosed herein.